The present invention relates to a seat reclining mechanism for vehicles for adjusting an inclination angle of a seat back with respect to a seat cushion, and more particularly to a seat reclining mechanism for vehicles having a limiting means which is mounted between a ratchet and a pawl each secured to one of the seat back and the seat cushion, respectively.
A known seat-reclining mechanism of this kind is described in Japanese Patent Laid-Open Publication No. Hei 9-154658. This device comprises a lower arm secured to a seat cushion on which a person sits, an upper arm secured to a seat back and rotatably held to the lower arm, a ratchet mounted on the upper arm, and a pawl slidably supported on the lower arm, in such a manner that when the pawl slides in a radial direction, it can come into and out of engagement with the ratchet. An operation lever or handle is linked to the pawl via a cam. The pawl can be brought into and out of engagement with the ratchet by operating the lever so as to slide the pawl over the lower arm. The rotation of the upper arm relative to the lower arm is limited by engagement of the pawl with the ratchet. The seat back is held at a given angle to the seat cushion. Rotation of the upper arm relative to the lower arm is permitted by disengaging the pawl from the ratchet. In this way, the inclination angle between the seat back and the seat cushion of the seat can be adjusted.
The ratchet of this conventional device is fitted with internal teeth in mesh with external teeth of the pawl. The ratchet further includes a non-toothed portion with which the external teeth of the pawl come into sliding contact. The internal teeth set a range in which the upper arm can rotate relative to the lower arm. The non-toothed portion sets a range in which the upper arm can rotate freely relative to the lower arm, e.g., a position in which the seat back is tilted forwardly fully about the seat cushion of the seat.
The presence of the conventional device described above presents problems. During rotation of the seat back relative to the seat cushion of the seat, the pawl makes a transition from a position where the pawl faces the internal teeth of the ratchet to a position where the pawl faces the non-toothed portion. During this movement of the pawl, the external teeth of the pawl might interfere with the ratchet at the interface step between the internal teeth of the ratchet and the non-toothed portion, depending on the position of the sliding pawl operated by the operation lever. Conversely, when the pawl moves from the position where it faces the non-toothed portion into the position where the pawl faces the internal teeth, the external teeth of the pawl come into sliding contact with the non-toothed portion of the ratchet. As a result, there arises a possibility of producing non-uniform wear.